The preferred embodiment concerns a device and method for inking of a charge image with toner material in a printer or copier. The surface of a photoconductor element comprises a photoconductive layer on which a charge image can be generated corresponding to a print image to be generated. The device also comprises an applicator element on whose surface a layer made up of toner particles has been applied in at least one region.
Developer stations in known printer or copier systems for development of charge images generated on a photoconductor, i.e. for development of latent print images, are used for implementation of image development methods in which the charge image is inked with toner across a gap. Such image development methods are known, for example, from U.S. Pat. No. 4,383,497. In known developer stations as they are often used in high-capacity printers, applicator elements are frequently provided in order to pass toner material past the charge image to be developed. The applicator elements are advantageously applicator rollers or continuous belts. A uniformly thick layer made up of toner particles that are charged and electrostatically adhere to the surface of the applicator element is applied on the surface of the applicator element in the developer station.
The charge image is located on a photoconductor, for example a photoconductor band or a photoconductor drum. The regions of the charge image to be inked are inked with the toner particles present on the surface of the applicator element. With the aid of an alternating voltage applied between the photoconductor and the applicator element, the toner particles electrostatically adhering to the surface of the applicator element are released from the surface of the applicator element in the region before, after and in the air gap via the alternating field generated with the aid of the alternating voltage, whereby what is known as a toner cloud is generated in this region. Due to a constant electrical field superimposed on the alternating electrical field, a force in the direction of the photoconductor is exerted on the toner particles contained in the toner cloud, whereby the toner particles are applied on the photoconductor corresponding to a charge image present there.
A concentrated application of toner material in the boundary regions of regions to be inked occurs in particular due to an excess supply of toner material in the toner cloud in the contact region between applicator element and photoconductor. Given a subsequent transfer and fixing of a toner image with boundary regions inked in a concentrated manner on a carrier material, a different optical density is generated in these boundary regions than in the rest of the regions to be inked.
The boundary regions inked in a concentrated manner are compacted upon transfer printing of the toner image from the photoconductor onto an intermediate carrier and upon transfer printing from this intermediate carrier onto a carrier material or from the photoconductor directly onto a carrier material, whereby adhesive forces are increased between the contact surfaces. These adhesive forces can pull toner material out from the boundary regions of the transferred toner image, which toner material than remains on the photoconductor or on the intermediate carrier. Toner material can thereby also be pulled out from the inner regions to be inked. Given transfer printing onto white paper, white areas (for example in characters to be inked) are created due to this pulled-out toner material. Due to their un-inked points inside the characters, such characters are also designated as hollow characters. The presence of un-inked regions is also designated as canes.
Given printers with high printing speed, the developer rollers must also be adjusted exactly in order to not select the distance to be bridged too large on the one hand and, on the other hand, to still keep the spraying of toner material (due to the air flows occurring in the gap) in a range in which only slight visible effects on the inked toner image result. In particular in high-capacity printers, circumferential speeds of the photoconductor element and of the applicator element in the range of >=1 m/s are typical. Toner particles can also remain adhered on the regions that are not to be inked due to the mentioned air currents, due to the generated toner cloud and due to the toner particles additionally applied in the boundary regions, which lead to what is known as spraying of the toner material. This spraying is also designated as spreading.
In order to also enable the exact adjustment of the gap between applicator element and photoconductor, highly-precise and exactly-adjustable components must be used, whereby a significant adjustment effort is additionally required. Particles and foreign bodies that are thicker than the developer gap lead at the least to the destruction of the photoconductor since both the position of the photoconductor and the position of the applicator element in the transfer region are fixed and the developer gap is mechanically limited.
The concentrated application of toner material in the boundary regions of regions to be inked also leads to more toner material being consumed than is absolutely necessary for generation of a high-quality print image.
Arrangements and methods for inking of charge images with toner material are known from the documents DE 43 42 060 A1, DE 103 54 347 A1, EP 1 213 621 A1, EP 1 154 332 and EP 1 178 361 A2.